1. Field of the Invention
This invention relates to a liquid crystal compound, more particularly to a liquid crystal compound having negative dielectric anisotropy, and a liquid crystal composition including the same.
2. Description of the Related Art
Compared with a twisted nematic liquid crystal display unit, a vertical alignment liquid crystal display unit has the advantages of wider viewing angle, higher contrast, faster response time, etc. A liquid crystal compound used in such a vertical alignment liquid crystal display unit should have negative dielectric anisotropy (Δ∈), good light and heat stability, a broad nematic temperature range, fast response time, etc.
In order to have a broad nematic temperature range, it is desirable to lower the crystal-to-nematic transition temperature (Tc), and to increase the nematic-to-liquid transition temperature (Tni). Besides, if a liquid crystal compound has an increased refractive index anisotropy (Δn), a cell gap for filling with liquid crystal can be reduced. Faster response time can be achieved by reducing rotational viscosity (γ1) and increasing the absolute value of Δ∈. A conventional liquid crystal compound having negative dielectric anisotropy generally has a 2-3-difluorophenyl group. However, although such a conventional liquid crystal compound has good light and heat stability, it has a relatively high rotational viscosity and low refractive index anisotropy.
For example, U.S. patent application publication no. 2011/0089374 A1 discloses two liquid crystal compounds respectively represented by the following formulas (A) and (B) (also see structures I-44 and I-45 of the U.S. publication).

where R11 and R13 are each independently C1-C7 n-alkyl or C1-C7 alkyloxy; R12 and R14 are each independently H or C1-C7 n-alkyl; X11, X12, X15 and X16 are each independently selected from the group consisting of F, Cl, CF3, CHF2, OCF3 and OCF2H; and X13 and X14 are each independently H or F. It is disclosed in said U.S. publication that the two liquid crystal compounds of formulas (A) and (B) have negative dielectric anisotropy. However, specific examples of the two liquid crystal compounds and other properties of the two liquid crystal compounds (such as refractive index anisotropy, nematic temperature range, etc.) are not disclosed therein.
It would be helpful to the development of vertical alignment liquid crystal displays if a liquid crystal compound having negative dielectric anisotropy, suitable refractive index anisotropy, and a wide nematic temperature range can be developed.